Frequently, when producing hydrocarbon fluids, e.g., oil and/or gas, from a formation, solids are produced along with the fluids. These solids can range in particle size from very fine silt to very coarse grained material, depending on the nature of the formation. Formations that produce solids vary from totally unconsolidated (uncemented) to weakly cemented. Formations having significant compressive strength of about 500 psi or greater, do not produce solids under normal operating conditions.
Various techniques are employed for controlling the production of these solids. One such technique is called gravel packing. Gravel packing involves filling an annulus or annular space between a casing and a retaining screen with a sieved particulate such as sand, the casing having been previously perforated. For best results for well productivity, sand also is placed into and through the perforation tunnels using pumping techniques. Subsequently, as the well is produced, sand serves as a filter media to restrain the movement and production of formation solids. The screen, in turn, prevents the movement of the sieved sand or "gravel".
In the practice of gravel packing, the major restriction to flow occurs in "gravel" filled perforation tunnels. This restriction is minimized by utilizing as large a perforation density as is practical and appropriate. For example, in conventional completions where gravel packing is not used, perforation densities rarely exceed four shots per foot (SPF) and are frequently less. In gravel packing operations, perforation densities are commonly 8-16 SPF.
When performing gravel packing operations, sand or "gravel" is mixed with an appropriate fluid into a slurry and pumped down the wellbore in a manner designed to fill the perforation tunnels and any voids that might exist outside the casing. Also, of course, the annular space between casing and retaining screen is filled. While successful in the majority of applications, gravel packs frequently fail to control solids production. A prime cause of failures occurs when the spaces designed to be filled with "gravel" are incompletely packed for one reason or another. As a result, voids are left in the pack. During subsequent production, formation solids are produced through them. For these reasons, placement of gravel becomes a major operational consideration in achieving successful gravel packs.
Therefore, what is needed is a method for effectively gravel packing a wellbore which packing will fill all desired spaces.